The invention relates to a compound chemically sensitive element, and more particularly, to such element which may be used to determine the concentration of a specific ion or other specific substance contained in a solution to be examined such as blood serum.
As is recognized, an ion sensor having a selective ion sensitive electrode is used in a determination of the concentration of a specific ion in an electrolytic solution. A glass electrode which is used in a pH meter is a typical example. An ion sensor having a selective ion sensitive electrode advantageously permits a continuous determination of the concentration of a specific ion in an electrolytic solution in a very simple manner by merely immersing the sensor directly into the solution to determine an electric potential.
Recently, with an increasing demand on the miniaturization of such ion sensor, a variety of chemically sensitive elements have been proposed which comprise a chemically sensitive film of one kind or another formed on the gate of an insulated gate field effect transistor (IGFET). (cf. IEEE Transactions on Biomedical Engineering, Vol. BME-19, No. 5, September 1972, an article by P. Bergveld on page 342.) A chemically sensitive element which is based on the principle of such IGFET element is generally referred to as ion sensitive field effect transistor (ISFET). A variety of chemically sensitive elements are available which are capable of detecting the presence of various ions and other substances depending on the kind of a chemically sensitive film which is applied to the gate thereof.
By way of example, Japanese Laid-Open Patent Application No. 26292/1977 discloses a pH sensor comprising SiO.sub.2 -SiN applied to the gate of a MOS (metal oxide semiconductor) transistor. The pH sensor can be manufactured utilizing a conventional process of manufacturing a semiconductor device, and hence is easily manufactured. It exhibits an excellent functionability and durability. Various ion sensors are proposed which comprise the pH sensor having SiO.sub.2 -SiN layer applied to its gate and having an ion sensitive film which is responsive to a specific ion formed on the layer.
An ion sensitive film conventionally used in the art of ion selective electrodes comprises a high melting glass film, but it is very difficult to apply such film to the gate of ISFET to which SiO.sub.2 -SiN layer is already formed. For example, a film which is sensitive to Na.sup.+ comprises soda aluminosilicate glass (11% NaO.sub.2 --18% Al.sub.2 O.sub.3 --71% SiO.sub.2), and has a high viscosity as well as a vitrifying temperature which is as high as 1,300.degree. C. or even higher. It will be seen that it will be very difficult to form such an ion sensitive film directly on the gate of MOS transistor as by chemical vapor deposition (CVD) or sputtering without destructing the function of the transistor. To accommodate for this difficulty, metal alcoholate is utilized to form a solution having a given proportion of glass components, and the solution is uniformly applied to the upper surface of SiO.sub.2 -SiN layer on the gate of ISFET either by spraying, coating, dipping or like process. Subsequently, it is heat treated at a low temperature which is equal to or below 500.degree. C. to form an ion sensitive film. However, the film formed according to this process has a durability which is greatly inferior to that of soda aluminosilicate glass which is used as Na.sup.+ sensitive film or soda aluminosilicate glass (27% NaO.sub.2 --5% Al.sub.2 O.sub.3 --68% SiO.sub.2) which is used as K.sup.+ sensitive film. For these reasons, an ion sensor having an ion sensitive film formed on the gate of ISFET which comprises SiO.sub.2 -SiN layer is limited in the choice of material which forms the ion sensitive film, and exhibits a durability and ion selectivity which are less than desired.
To eliminate such disadvantages of the conventional ion sensor, the present applicant has previously proposed an ion sensor in which an ion sensitive assembly which is sensitive to a specific ion in a solution to be examined and the insulated gate of a pH sensor are electrochemically connected together by an electrolytic solution or a gel-like material containing an electrolyte (see Japanese Patent Application No. 66,092/1979). With this ion sensor, the ion sensitive assembly can be manufactured separately from the pH sensor, and hence its manufacture is facilitated. In addition, any known material having excellent durability and ion selectivity may be freely used for the ion selective film. However, in the disclosed ion sensor, the connection of the ion sensitive assembly with the insulated gate of the pH sensor by means of an electrolytic solution or a gel-like material containing an electrolyte prevents stable use over a prolonged period of time due to the evaporation or desiccation of the electrolytic solution or the gel-like material. Thus, after a given period of use, the electrolytic solution or the gel-like material must be replenished, resulting in troublesome maintenance and control. Also, in the actual determination, the ion sensitive assembly and the insulated gate of the pH sensor must be positively connected together by the electrolytic solution or the gel-like material at the same time as the ion sensitive assembly is brought into contact with a solution to be examined, and this limits the orientation in which the ion sensor is disposed, again resulting in a troublesome handling.